Michael J. Day, BSc, BVMS (Hons), PhD, DSc, DECVP, FASM, FRCPath, FRCVS
One of the most frequently asked questions related to adverse reactions following vaccination is that of how to approach revaccination of an animal in which vaccination was suspected as acting as a trigger for the adverse event. The answer to this question is not straightforward and requires consideration of the circumstances of the individual animal in discussion with the client.
Did the Vaccine Cause the Problem?
The first consideration in such cases is whether the vaccination was actually the primary trigger for the observed adverse event. In some instances this is relatively clear (e.g., an animal develops facial oedema and pruritus within 15 minutes of vaccination), but in others the cause-effect relationship may not be so obvious (e.g., a dog develops immune-mediated haemolytic anaemia [IMHA] four weeks after vaccination; a cat develops an interscapular fibrosarcoma 5 years after last being vaccinated at that site). A wide range of suspected adverse reactions are reported following vaccination of dogs and cats and it is generally accepted that particular events occurring within a 4 week period post-vaccination might be considered as potential vaccine-associated reactions. It is important to remember that the incidence of adverse reactions following vaccination in dogs and cats is relatively low (around 50 per 10,000 animals vaccinated) and the benefit of vaccination (in protecting the individual and the population from life-threatening infectious disease) far outweighs the risk of adverse reactions. The majority of adverse reactions are also relatively mild and non-life threatening.
A 6-year-old, female English cocker spaniel dog develops IMHA two weeks after receiving a multi-component booster vaccination (DHPPiL in a non-rabies endemic country). The animal is successfully treated with immunosuppressive doses of oral prednisolone and is now clinically normal. The owner asks for your advice on how this dog should be revaccinated in the future.
Is There a Risk?
One of the hallmarks of immune-mediated disease is of a relapsing remitting history. Such relapses can be severe or other immune-mediated events may occur. This dog is at risk of disease relapse. If we believe vaccination to have been a trigger event in this case, then revaccination may theoretically be associated with the potential for disease relapse. The immunological mechanisms by which vaccination triggers a disease such as IMHA are unknown, but it is likely that immunological sensitization and hypersensitivity have a role.
How Do I Reduce the Risk?
If revaccination carries a risk of triggering a relapse of IMHA in this dog, then clearly minimizing the future vaccine load in this animal will reduce such risk. This then becomes a 'risk-benefit analysis' conducted with the client and considering the lifestyle of the animal, its risk of exposure to infectious disease and the background legal framework regarding vaccination.
The first consideration is whether this dog requires revaccination at all. This is an adult dog that was appropriately immunized as a pup and received DHP boosters at 3 and 6 years with LPi boosters annually. Although the licensed duration of immunity (DOI) for the core vaccine components (DHP) is three years, there is now evidence for a minimum DOI of 9 years for CDV and CPV and, in reality, a dog that is appropriately immunized as a pup probably never requires another core vaccine during its lifetime. The non-core components of this animal's vaccine schedule (LPi) are also unnecessary. Although they do not have a DOI greater than 1 year, this is a city dog that is never kennelled in a boarding establishment and its lifestyle means that its risk of exposure to Leptospira or the canine respiratory complex is minimal. If the owner is in any doubt as to whether the animal is protected against the core vaccine-preventable diseases, then serological testing may be used to allay any fears. The presence of any titre of antibody to CDV, CAV and CPV is indicative of protection. If this strategy (of not revaccinating this animal) is adopted, then it is important that this is documented in the case record as 'informed client consent' (signed by the client) remembering that the legal (but not scientific) requirement is for revaccination every three years.
Should this client not accept these proposals, or should there be legal requirements that the dog is vaccinated (e.g., if the dogs lives in a country where rabies vaccination is mandatory or if the dog is regularly kennelled where 'current' vaccination is required), then it is still possible to reduce risk by using products with the longest possible DOI and by minimizing the use of non-core vaccines. Although the mechanisms by which vaccination might trigger IMHA are unknown, it also makes some sense to switch the brand of vaccine if revaccination is considered essential.